A simplified method for identifying the predominant clay mineral in soil



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Texas Tech University


Increased construction activity in sites that contain very active clay minerals has greatly expanded the necessity for engineering knowledge related to the type and amount of clay minerals in a given soil. Presently, there are varying methods of predominant clay mineral identification. These methods are, however, frequently time consuming and laborious and require expensive equipment that is not commonly found in the ordinary commercial soils testing laboratories.

Pearring in 1968 and, later, Holt in 1970 developed a correlation chart to aid in the identification of the predominant clay mineral in a given soil. The two parameters involved are Cation Exchange Activity and Activity Ratio. These two parameters require the plasticity index, the cation exchange capacity, and the percent of clay in the soil fraction passing the No. 200 sieve. Presently, cation exchange capacity determination is not devoid of expense and time problems. These problems prompted this research which is intended to relate the cation exchange capacity of a clay soil to the easily obtainable Atterberg limits (plastic limit and liquid limit) and plasticity index.

A detailed study was made of selected soils of varying geographic locations and geologic origins to establish data related to the chemical (cation exchange capacities) and engineering index properties of such materials. A study of the test results discloses that it is possible to predict the cation exchange capacity of a soil and, hence, the predominant clay mineral in the soil, using the plastic limit. The result of the correlation study shows a strong relation to exist between the cation exchange capacity and the plastic limit of all soils tested. This relation may be approximated by the expression CEC = PL^1.17.